1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to a system and method of amplifying an optical signal using multiple backward pumping systems.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables reliably transport optical signals over long distances. Over a distance, an optical signal attenuates in the fiber due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier, however, the optical amplifier adds noise to the optical signal. The noise accumulation on the optical signal can especially be a problem for ultra long haul transmissions that have a high bit rate.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. Distributed amplifiers use the transmission fiber, that is carrying the optical signal, as a gain medium. Discrete amplifiers do not use transmission fiber as a gain medium, but use another type of fiber or component as the gain medium.
One type of discrete amplifier is an Erbium-Doped Fiber Amplifier (EDFA). In an EDFA, an Erbium-doped fiber receives optical signals from a transmission fiber. A pump laser transmits a 980 nm laser beam onto Erbium-doped fiber concurrently as the optical signals travel over the Erbium-doped fiber. The properties of the Erbium-doped fiber act to absorb the laser beam and generate a gain in the optical signals using the absorbed laser beam. In this example, the Erbium-doped fiber acts as the gain medium, not the transmission fiber. Unfortunately, EDFA's have a limit on the gain bandwidth they can produce and cannot effectively be used for ultra wide band transmissions.
Another type of discrete optical amplifier is a Raman amplifier. In a discrete Raman amplifier, a fiber span within the Raman amplifier receives optical signals from a transmission fiber. The fiber span may be a highly doped fiber, such as a dispersion compensating fiber. A Raman pump laser backward pumps a laser beam onto the fiber span carrying the optical signals. Based on the “Raman Effect”, the laser beam generates a gain in the optical signals traveling on the fiber span. For instance, a 1480 nm laser beam, transmitted over a fiber span carrying optical signals, generates a gain in the optical signals in the range of 1565–1600 nm. The discrete Raman amplifier provides a wider gain bandwidth and allows for replacement of high-powered EDFAs. However, the discrete Raman amplifier generates a higher noise figure than EDFAs.
Raman pump lasers can also be used for distributed amplification. Designers have improved the noise figure problems for distributed Raman amplification using a second order pump. One particular pumping scheme for a distributed Raman amplifier was described in a paper entitled “1.6 Terabit/s (160×10.66 Gbits/s) Unrepeatered Transmission Over 321 km Using Second Order Pumping Distributed Raman Amplification”, which was authored by L. Labrunie et.al. and published in OAA '01, PD-3 (Labrunie paper), which is incorporated herein by reference into this application. This scheme involves backward pumping two first order pumps onto a fiber span and also backward pumping a second order pump onto the fiber span. One of the first order pumps has a power of 230 mW at 1425 nm, and the other first order pump has a power of 150 mW at 1455 nm. The second order pump has a power of 1.4 W at 1367 nm. The two first order pumps and the second order pump generate a gain in the optical signals. This configuration is illustrated below in FIG. 1.
One problem with the configuration in the Labrunie paper is that the first order pumps are set to high powers (230 mW and 150 mW), and the first order pumps generate a gain in the optical signals close to a receiver end of the fiber span. The receiver end is the end of the fiber span from which the optical signals exit the fiber span. In generating the gain near the receiver end, the first order pumps also generate noise in the optical signals near the receiver end. Generating the noise near the receiver end can negatively affect the Optical Signal-to-Noise Ratio (OSNR) of the optical signals.